In addition to the usual forms of high molecular weight DNA found in mitochondria (closed circles, open circles, linear duplexes), we have recently discovered another form of DNA of lower molecular weight possessing an s value of about S8. Evidence has been obtained that this DNA is neither an artifact nor a nuclear contaminant, and does not result from the breakdown of high molecular weight DNA. We have concluded that this DNA is a non-trivial component of the mitochondrion and are investigating its possible function.